It may be a Configurate issue. Configurate is the library to use for configs in Sponge. A new version of Configurate(3.2) is used since API 5, commit 0145f2cabdd13289cff0440bda0c9568deec285e.
SpongeForge uses it since commit 65019f76a78d243d8d80ff64d94a09855a02fe40.
SpongeVanilla since commit 42038102d5ee37c3e7541abcd7cf5405ba3f111a.
Use a build before that commit; SpongeVanilla build
1.10.2-5.0.0-BETA-137 or before; SpongeForge: I think there is no easy way to find it out, you would have to download the SpongeForge builds and compare their commits with the commit mentioned above.
Just try an older build of Sponge until I have figured out if it is a Configurate bug.